avcodec/aacenc: Check both channels for finiteness

Fixes null pointer dereference
Fixes: 10412fc52ecc6eab40ed67f82ca7b372/signal_sigsegv_2618c99_2129_f808373959e46afb165593332799ffbc.aif

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/aacenc.c

@@ -607,14 +607,14 @@ static int aac_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
                 s->mdct1024.mdct_calc(&s->mdct1024, sce->lcoeffs, sce->ret_buf);
             }
 
-            if (!(isfinite(cpe->ch->coeffs[    0]) &&
-                  isfinite(cpe->ch->coeffs[  128]) &&
-                  isfinite(cpe->ch->coeffs[2*128]) &&
-                  isfinite(cpe->ch->coeffs[3*128]) &&
-                  isfinite(cpe->ch->coeffs[4*128]) &&
-                  isfinite(cpe->ch->coeffs[5*128]) &&
-                  isfinite(cpe->ch->coeffs[6*128]) &&
-                  isfinite(cpe->ch->coeffs[7*128]))
+            if (!(isfinite(cpe->ch[ch].coeffs[    0]) &&
+                  isfinite(cpe->ch[ch].coeffs[  128]) &&
+                  isfinite(cpe->ch[ch].coeffs[2*128]) &&
+                  isfinite(cpe->ch[ch].coeffs[3*128]) &&
+                  isfinite(cpe->ch[ch].coeffs[4*128]) &&
+                  isfinite(cpe->ch[ch].coeffs[5*128]) &&
+                  isfinite(cpe->ch[ch].coeffs[6*128]) &&
+                  isfinite(cpe->ch[ch].coeffs[7*128]))
             ) {
                 av_log(avctx, AV_LOG_ERROR, "Input contains NaN/+-Inf\n");
                 return AVERROR(EINVAL);